This invention relates to a process for manufacturing polyvinyl chloride compositions having improved properties such as, for example, heat distortion temperatures.
Polyvinyl chloride, sometimes hereinafter abbreviated to PVC, is an established synthetic resin with numerous applications, including components for the construction industry such as house sidings and window frames, water pipes, toys, and various household articles. PVC is a hard and brittle resin, which normally is not used as such but is compounded with processing aids, plasticizing polymers and/or liquid plasticizers, and stabilizers, which improve its processability and performance. Uncompounded PVC has a heat distortion temperature (HDT) of about 80.degree. C., but commercially available compounded rigid PVC has an HDT of only about 60.degree.-70.degree. C. Some articles, where rigid PVC either is or could be used, such as building components and appliance and computer housings are subjected either at certain periods of the year or usually to intense heat caused by their exposure to the sun or by the operation of the equipment housed therein. In order for PVC to be useful in such applications, it is, therefore, important to be able to increase the HDT of compounded PVC resins above its current levels.
PVC offers a considerable price advantage over other engineering resins, but its use as a structural material has been rather limited because of its low HDT. Methods of increasing its HDT frequently also lower its impact resistance below acceptable limits.
It, therefore, would also be important for such uses to increase the HDT of PVC without significantly lowering its impact resistance. This currently can be done in two ways.
One is to add an incompatible resin having a sufficiently high glass transition temperature (Tg), e.g., higher than 130.degree. C. and a flexural modulus of more than about 690 MPa, such as, e.g., a polycarbonate or a polysulfone resin or an inorganic filler, e.g., glass fibers, glass beads, or titanium dioxide particles. Addition of inorganic fillers suffers from the drawback that it rapidly increases the viscosity of the blend to a point of difficult workability or complete unworkability. But in either case, the maximum HDT that can be attained in this manner is about 80.degree. C., that is, the HDT of uncompounded PVC.
The other method is to add a resin having a sufficiently high Tg and flexural modulus that is miscible with PVC. This can result in blends having an HDT higher than 80.degree. C. Some work addressing this approach has been reported in patent literature.
U.S. Pat. No. 4,255,322 to Kopchik discloses blends of PVC with polyglutarimides, which preferably also contain a third polymer serving as impact modifier. The patentee determined that PVC and polyglutarimides are compatible, and, further, that the blends have improved HDT's. The polyglutarimides themselves, also known as imidized acrylic resins or imides of polyacrylic acids, were already described by Graves in U.S. Pat. No. 2,146,209, Schroeder in U.S. Pat. No. 3,284,425, Barabas et al. in U.S. Pat. No. 4,169,924, and Kopchik, U.S. Pat. No. 4,246,374. Certain details of the experimental work described in the Kopchik '322 patent are unclear. It is, therefore, not possible to determine by reading the examples of that patent how the components of the final compositions were blended together, although it appears that this was always done in one step. Further, the properties of the blends of Example 7 of that patent, which were supposed to o be given in Table III, are not given there or anywhere else.
U.S. Pat. No. 4,595,727 to Doak improves on Kopchik by employing a rubber-modified PVC, rather than blends containing a separate elastomeric impact modifier. Doak reports an HDT for a blend of rubber-modified PVC with 30% of polyglutarimide of 88.degree. C. When the amount of polyglutarimide is increased to 60%, the HDT is increased to 105.degree. C.
U.S. Pat. No. 3,629,170 to Yamanouchi et al. also discloses improved PVC compositions. The improvement is obtained by solution-blending PVC with a polysulfone resin and evaporating the solvent. This reference does not report the HDT but only the Vicat softening temperature of the resulting blends; but, even if an improvement is obtained, it is clear that solution blending is not a practical industrial way of making polymer blends. In any event, since polysulfones are incompatible with PVC, it is not expected that a significant HDT improvement could be obtained in this way.
Rohm & Haas Company, the assignee of the Kopchik patents, is offering a broad range of polyglutarimides for various uses, including a grade designated as PARALOID.RTM. HT-510 for blending with PVC and a family of higher Tg resins designated as PARALOID.RTM. EXL-4000 for blending with other engineering resins.
It is customary to melt-blend polymers on an industrial scale in continuous equipment operating at an acceptably high temperature with a short residence time. Such equipment normally would be an extruder, although various blenders or mixers also may be used. Extruders are the most convenient to use because of their high throughput, possible modular construction and ease of assembly, choice of many mixing screws, and ease of control and maintenance of process temperatures.
A resin such as a Rohm & Haas PARALOID.RTM. HT-510, which has a fairly low Tg, determined by the present inventor to be about 130.degree. C., can be melt-blended with PVC in an extruder. Since, as a rule of thumb, the processing temperature is at least about 100.degree. C. above the Tg, this melt-blending temperature would be at least 230.degree. C. However, PVC begins to decompose above about 210.degree.-220.degree. C., and its rate of decomposition above about 230.degree. C. is quite rapid and further increases with temperature. Accordingly, while it is possible to directly melt-extrude such blends, those high melt processing temperatures are undesirable in the industry, where it is desired to operate within the temperature range of 150.degree.-220.degree. C., preferably 150.degree.-210.degree. C. Temperatures up to about 230.degree. C. may sometimes be tolerated but normally are not recommended. Even if a good, homogenous blend is obtained at those higher temperatures, discoloration, hydrogen chloride evolution, or other signs or decomposition frequently will be observed. Still, the amount of decomposition depends not only on the temperature but also on the residence time. A temperature of 230.degree. C. may be quite acceptable for a residence time of less than a minute but not for several minutes.
If the same resin with a Tg of 130.degree. C. is blended with PVC below 220.degree. C., a complete homogeneous dispersion often is not obtained and the HDT improvement is small, usually not exceeding by much the HDT of uncompounded PVC. The situation is even more complicated when PVC, polyglutarimide, and a toughening polymer are blended together in an extruder. In addition to the fact that a complete dispersion of polyglutarimide in PVC usually is not obtained, the resulting HDT of the blend is lower than it would be in the absence of the toughening polymer, which normally is a low Tg material. The flexural (or flex) modulus of such compositions also is lower than it would be in the absence of the toughening polymer.
As the Tg of the polyglutarimide increases, it is much more difficult or impossible to directly melt-blend it with PVC under conditions normally employed in the industry. The PARALOID.RTM. EXL-4000 family has Tg values reported by the manufacturer as being in the range of 140.degree. C. to 170.degree. C. These resins are offered by Rohm & Haas for blending with industrial resins such as, e.g., nylon 6, polycarbonates, acrylonitrile/styrene/butadiene and styrene/acrylonitrile resins, and poly(ethylene terephthalate) to increase their heat resistance or melt strength, to improve optical properties, or to serve as carriers for pigments and other additives. Yet, they would be potentially capable of increasing the HDT of PVC to a much higher extent than can be obtained with PARALOID.RTM. HT-510 and, because they also have higher flex moduli, the resulting blends can be expected to be more rigid even when containing a toughener. Unfortunately, nobody has proposed a process for making such blends.
There is, therefore, a need for an industrially practical process for producing toughened PVC compositions with improved HDT values and acceptably high flex moduli.